1. Field of the invention
The present invention relates to a photoelectric conversion device, particularly, a photoelectric conversion device for office appliances such as a copying machine, a facsimile, a scanner, or the like and a photoelectric conversion device to be used for an x-ray camera tube apparatus for medical use to be used for diagnosis or to be used for nondestructive inspection apparatus; an image processing (image information or image data processing) system; and their driving method.
2. Related Background Art
Conventionally, x-ray direct radiography has been carried out by a so-called film way, which has been a mainstream method carried out by radiating x-rays to a patient and transferring the transmitted x-rays to a photosensitive film through visible light conversion phosphors. The film way has problems that it takes a long time from photographing to developing processes and that it requires to store and search an immense number of photographed films in terms of maintenance and management in a hospital.
By the way, there is another way available using extremely bright phosphors in place of the film which is carried out by storing an x-ray image of a patient in the extremely bright phosphors, scanning the image with a laser beam thereafter, and reading the x-ray image as digital values. If an image is converted to be digital, that makes recording in a variety of media possible, so that image storage, search, and transfer can easily be carried out to improve the efficiency in a hospital in terms of the maintenance and management. Further, obtaining image data as digital values makes highly advanced image processing by a computer possible and is expected to result in improvement of diagnosis.
However, even the way using extremely bright phosphors has, as in the film way, a problem that it takes a long time from photographing to developing.
On the other hand, an x-ray imaging apparatus using a solid imaging (image-pickup) device such as CCD and an amorphous silicon semiconductor has been proposed. As in the film way, it is carried out by directly converting an x-ray image of a patient obtained through visible conversion type phosphors by x-ray radiation to a digital image, approximately in real time, by a large number of imaging (image-pickup) elements arranged in a two-dimensional array and reading the image. Since the digital image can be obtained approximately in real time, as compared with the above described film way and the way using the extremely bright phosphors, it is greatly advantageous. Especially, since amorphous silicon can be formed in a large surface area, in the case of an x-ray imaging apparatus using such amorphous silicon, photographing (image-pickup) of a large portion such as chest photography can be carried out in equal size. Consequently, a high light utilization factor is provided and a high S/N ratio is expected.
However, in an x-ray imaging apparatus for medical use, in order to photograph a chest of a human being in equal size, it is required to make available a solid imaging device with a large surface area as wide as 40 cm×40 cm and a number of pixels as extremely high as 5,000,000 to 10,000,000.
It is not so easy to produce the properties of these immense numbers of pixels all evenly and reliably. For example, in the case of using amorphous silicon for photoelectric conversion elements and switching elements for charge transfer, if continuous operation is carried out for a long duration, dark current of the photoelectric conversion elements will be increased and the properties of the switching elements will be changed. To deal with such cases, a countermeasure employed is that the photoelectric conversion elements and the switching elements are so designed as to be operated only at the time of taking images and so as to be inhibited from operation at the time of taking no image. For example, when no patient is in a photographing chamber, bias wires of the photoelectric conversion elements and gate wires and reading-out wires of the switching elements are biased to be at zero potential, and no electric field is applied to the insides of the amorphous elements to lower the property alteration of the elements in a long time use. However, this case has a complicated handling of operating the photoelectric conversion elements and switching elements after recognizing the presence of a patient near the imaging apparatus and then carrying out an image pickup operation. In other words, operability of the apparatus is deteriorated. A design to automatically recognize the existence of a patient may also be possible; however, it leads to an increase in cost of an apparatus.
On the other hand, in the case where a large number of photoelectric conversion elements with a large surface area is fabricated using an amorphous silicon thin film, there are problems that traces of impurities are mixed in fabrication processes and dangling bonds tend to be increased and that they form as defect levels. They work as trapping levels and become unnecessary dark current in the photoelectric conversion process to decrease the S/N ratio. As a driving method of a photoelectric conversion device in which the dark current is lessened, possible is a method of carrying out photoelectric conversion after the dark current is moderated by waiting for several to several 10 seconds from biasing the photoelectric conversion elements (and switching elements). However, if such a method is employed for an x-ray imaging apparatus, the cycle for taking images of a plurality of patients will become long.
As described above, in the photoelectric conversion device with a large surface area using amorphous silicon, it is made difficult to keep a high S/N ratio owing to the property alteration during a long time use and defect levels in a film and hence, a photoelectric conversion device and its driving method capable of being operated easily while keeping good S/N ratio have been expected to be developed.